The xanthophyll cycle, its regulation and components

During the last few years much interest has been focused on the photoprotective role of zeaxanthin. In excessive light zeaxanthin is rapidly formed in the xanthophyll cycle from violaxanthin, via the intermediate antheraxanthin, a reaction reversed in the dark. The role of zeaxanthin and the xanthophyll cycle in photoprotection, is based on fluorescence quenching measurements, and in many studies a good correlation to the amount of zeaxanthin (and antheraxanthin) has been found. Other suggested roles for the xanthophylls involve, protection against oxidative stress of lipids, participation in the blue light response, modulation of the membrane fluidity and regulation of abscisic acid synthesis. The enzyme violaxanthin de-epoxidase has recently been purified from spinach and lettuce as a 43-kDa protein. It was found as 1 molecule per 20-100 electron-transport chains. The gene has been cloned and sequenced from Lactuca sativa, Nicotiana tabacum and Arabidopsis thaliana. The transit peptide was characteristic of nuclear-encoded and lumen-localized proteins. The activity of violaxanthin de-epoxidase is controlled by the lumen pH. Thus, below pH 6.6 the enzyme binds to the thylakoid membrane. In addition ascorbate becomes protonated to ascorbic acid (pKa= 4.2) the true substrate (Km= 0.1 mM) for the violaxanthin de-epoxidase. We present arguments for an ascorbate transporter in the thylakoid membrane. The enzyme zeaxanthin epoxidase requires FAD as a cofactor and appears to use ferredoxin rather than NADPH as a reductant. The zeaxanthin epoxidase has not been isolated but the gene has been sequenced and a functional protein of 72.5 kDa has been expressed. The xanthophyll cycle pigments are almost evenly distributed in the thylakoid membrane and at least part of the pigments appears to be free in the lipid matrix where we conclude that the conversion by violaxanthin de-epoxidase occurs.